1. Field of the Invention
This invention is a technique for fabricating optical fibers.
2. Description of the Prior Art
The central problem in the fabrication of optical fibers of sufficient quality for use in long distance transmission systems involves the reduction of optical losses to tolerable levels--usually less than 10 db/km. The purity required for such low loss qualities is achieved by means of sophisticated deposition techniques used in fabricating the preforms from which the optical fibers are drawn.
There are currently two major techniques appropriate for commercial fabrication of optical fiber preforms--the "soot deposition" process, and the modified chemical vapor deposition technique (MCVD). Both of these processes involve the thermochemical production of glass from appropriate glass precursor vapors.
In the soot depositin technique, disclosed in U.S. Pat. Nos. 3,826,560 and 3,823,995, the glass precursor vapors are introduced into a hydrolyzing flame. In the ensuing reaction, particulate material--commonly referred to as soot--is formed. The stream of particulate material emanating from the flame is then directed toward a mandril, which may be, for example, a tube or a glass rod, to which the soot adheres. Following the deposition, the soot is consolidated into a transparent glass, the mandril may be removed, and the preform is drawn into a fiber. Although deposition rates using this technique are rapid, the nature of the flame reaction is such that the presence of impurities, particularly the hydroxyl radical, is a recurring problem.
The hydroxyl impurity problem, which is prevalent in the soot technique, may be more effectively limited in the various chemical vapor deposition techniques (CVD). In the simple CVD process, a stream of glass precursor vapors is directed through the center of a glass tube. The tube is usually composed of a glass material which may be appropriate for use as a cladding in the fiber. The tube is heated, causing the gas vapors to react at the walls of the tube--a heterogeneous reaction--forming directly an appropriate glass. The absence of a flame, and its associated hydroxyl impurities, accounts, in part, for the high glass purities and low optical losses obtained using the CVD technique. However, the CVD processes are normally too slow for practical commercial applications.
An invention which allows for the practical commercial application of a chemical vapor deposition process is described in a commonly-assigned application Ser. No. 444,705. In that application, it is shown that if the reactant concentration and the temperature to which the tube is heated are raised sufficiently above that which is normally encountered in the conventional CVD process, a new and more efficient process takes place--the modified chemical vapor deposition process (MCVD). Under these circumstances, the glass precursor vapor may be transformed into both a vitreous phase on the glass walls--a heterogeneous reaction--and a particulate phase in the center of the tube away from the walls--a homogeneous reaction. The particulate material "falls out" onto the tube walls and is consolidated into a glass, and onto the tube, as the heated zone traverses the length of the tube. This process has a much more rapid deposition rate than the simple CVD process. The impurities associated with the ignition, present in the hydrolysis burner of the soot technique, are not a consideration in the MCVD process. It is found that the integrity of the tube may be maintained throughout the MCVD process despite the high temperatures required for its effective implementation.